Modern spacecraft use a majority of their fuel supplies just to break away from the Earth's gravity and actually get into space. But a new NASA proposal hopes to cut that fuel requirement in half by employing solar-powered, ion-thrusting "Space Tugs" to ferry ships and satellites into Geosynchronous orbit.

These Space Tugs would be powered by a Solar Electric Propulsion (SEP) system—essentially a solar-powered ion thruster. Ion thrusters provide the same basic sort of movement as conventional thrusters but operate on an entirely different mechanism. Both propulsion types move a spacecraft by pushing it in the same way a stream of air pushes a balloon around as it deflates.

Chemical thrusters operate by mixing and burning liquid fuel with an on-board supply of oxygen. This is a method has a low specific impulse, or propellant efficiency. Ion thrusters instead generate their push from high-speed ions being ejected from an on-board supply of electrified xenon gas.

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Specifically, xenon atoms are injected into the craft's propulsion chamber where they are heated and electrified to create a plasma, which in turn generates xenon ions. The differently-charged ions are then separated with positive ions getting pushed to the rear of the chamber. From there, the positive ions are accelerated through an electrostatic away, at speeds of 30-100km/s, and are ejected from the rear of the spacecraft. This provides thrust—not nearly as much as chemical propulsion but with an exceedingly high specific impulse—that increases without abatement the longer the engine is run. A cathode on the rear of the ship then shoots electrons at the ion stream where they recombine with the positive ions into xenon atoms.

The downside of ion propulsion is its high energy requirement. The HiPEP thruster that was designed for the now defunct Jupiter Icy Moons Orbiter, for example, needed 20-50 kW to operate. Since batteries large enough to hold the necessary charge would also be prohibitively heavy, these thrusters can also employ solar arrays to continuously power the engines during their years-long operations.

Last year, NASA awarded $3 million in contracts to Analytical Mechanics Associates, Ball Aerospace & Technologies, Boeing, Lockheed Martin, and Northrop Grumman for a 4-month study of SEP systems for their use in ferrying payloads from Low Earth Orbit (LEO) up to the higher geosynchronous Earth orbit (GEO) and Earth-Moon Lagrange point one (EML 1). EML1 is the point where the relative gravitational forces of the Earth and the moon negate each other. This gravitational dead-zone is ideal for constructing large space-faring vessels and space stations like the Exploration Gateway Platform given the structures wouldn't fear atmospheric reentry.

Employing a reusable SEP-powered Space Tug to move payloads into higher orbits could provide significant saving to a space program. Second-stage chemical rockets would be rendered unnecessary and, given the SEP's energy-efficiency, the Tugs would require infrequent refueling.

Northrop Grumman has announced that it is working on an SEP system scalable to 300kW. This could prove very useful for exploring near-Earth asteroids as well as deep space. Aerojet, on the other hand, is eyeing SEP technology for the upcoming Orion project. "For example, a 25-40 kW SEP vehicle using current technology can pre-position a human-tended habitat at L-2 to support initial Orion missions. This approach would provide an immediate deep space destination for astronauts, and L-2 is an excellent way-station to the rest of the solar system," noted Julie Van Kleeck, Aerojet Vice President, Space & Launch System in an interview with NASASpaceflight.com. [Gizmag - Ion Thruster Wiki - NASA Spaceflight - Parabolic Arc - Electrostatic Ion Thruster Wiki]